The invention is directed to a method for generating ions from thermally unstable, non-volatile, large molecules, particularly for a mass spectrometer such as a time-of-flight mass spectrometer, whereby a specimen substance comprising the molecules is exposed to energy pulses by which molecules are released from the specimen substance, and whereby the released molecules are entrained by a jet of a carrier gas and are cooled upon expansion thereof and are subsequently ionized in an ionization chamber. The invention is also directed to an apparatus for generating ions from thermally unstable non-volatile, large molecules, particularly for a mass spectrometer such as a time-of-flight mass spectrometer, comprising a means for generating a carrier gas jet, an energy source for the desorbtion of molecules from the specimen material and comprising a means for introducing specimen material into the carrier gas jet, particularly for the implementation of the above-recited method.
German Letters Patent 38 00 504 discloses a method of the species wherein the desorbtion of the molecules ensues with a laser beam. It serves the purpose of converting, in particular, large molecules into the vapor phase before the molecules are brought by a subsequently implemented ionization process into a chemical condition wherein they become accessible for mass spectrometric analysis. What is thereby exploited is that the inner energy absorbed by the molecules due to the desorbtion is greatly reduced in the carrier gas jet, so that the molecules are intensively cooled and their thermal decomposition is largely prevented. This desorbtion process is suitable for liquid and solid specimen substances, whereby it has proven beneficial to accommodate the molecules of the specimen substance in a matrix that thermalitically decomposes easily.
An apparatus with this desorbtion process can be implemented is described in the periodical "ANGEWANDTE CHEMIE" 1988, pages 461 ff, in the overview article "Die Multiphotonen-Ionisation (MUPI) Massenspektrometrie". The specimen substance is thereby placed in front of the orifice of a nozzle from which the carrier gas emerges. By employing infrared laser light, the molecules of the specimen substance are desorbed into the expanding jet of the carrier gas. The inner degrees of freedom of the molecules are thereby cooled and the molecules are farther-conveyed by the carrier gas jet. This apparatus is usually operated as a pulsed system that is composed of a pulsed valve for producing the carrier gas jet and of a laser for the desorbtion of the neutral molecules. Since the molecules are farther-transported as a jet or, respectively, as a particle packet in pulsed mode, it is possible to keep this desorbtion process spatially separated from an ionization process that follows thereafter.
Single-photon or multi-photon ionization has proven itself for the mass-spectrometric examination of the large molecules under consideration. Since the wavelength of the beamed-in photons can be tuned to the energy difference between the basic condition and an excited condition of the neutral molecule, it is possible to undertake the ionization selectively vis-a-vis only the molecules under examination; the carrier gas particles thereby remain in a neutral condition and do not influence the subsequent examination results.
Although multi-photon ionization mass spectrometry is successfully carried out it can nonetheless not be employed for some problems since an only selective excitation of the neutral molecules can often not supply adequate information for the desired structural clarification of the molecule because the excitation wavelength to be selected can not be adequately predetermined given unknown molecules. It has also turned out that some substances can only be ionized with difficulty in the way set forth.
Ionization methods that act non-selectively are known. These include electron impact ionization. Such methods, however, cannot be employed for large molecules are in the present case since they lead to a great fragmentation of the molecule. Moreover, the carrier gas particles are also ionized, this leading to saturation effects, electrostatic repulsion and, thus, to poor resolution and inadequate sensitivity of the analysis. Such influences cannot be left out of consideration for the very reason that the carrier gas particles are present in a concentration that is at least a thousand-fold higher when compared to the molecules to be examined.
The employment of electron impact ionization is disclosed by German Letters Patent 873 765; the combination of this procedure with a method as known from German Published Application 36 19 886, however, only leads to highly fragmented ions in the low mass range, so that large, thermally unstable, non-volatile molecules such as, for example, peptides can thus not be examined therewith.